1. Field of the Invention
The present invention relates generally to a lens barrel and a camera system and, more particularly, to a lens barrel and a camera system for outputting data about a focal length of the lens barrel.
2. Related Background Art
According to a prior art camera system, items of focal length data and photographic distance data of a lens barrel are inputted to a camera body side, and an exposure, an autofocus adjustment and a light emission of an electronic flash are controlled based on the focal length data and the photographic distance data. For example, a photographic magnification .beta. and a field range 1 are obtained in accordance with formulae (1) and (2) by use of semi diagonal line length h of a photographic surface of a film from a focal length f and a photographic distance R. The obtained photographic magnification p is used for making a photometric calculation for controlling the exposure. The field range 1 is used for calculating an electronic flash irradiation angle for controlling the light emission of the electronic flash. EQU .beta.=f/R (1)
.beta.: photographic magnification,
f: focal length, and
R: photographic distance EQU 1=R.times.h/f (2)
1: field range, and
h: semi diagonal line length of a photographic surface of a film (21.62 mm in a 35 mm film)
Note that the following relationship is established from the formulae (1) and (2). EQU 1=h/p (3)
A nominal focal length is used for the focal length employed for the calculations in the formulae (1) and (2). The nominal focal length is a distance from a principal point to a focal point and, a light beam incident in parallel to the optical axis intersecting the optical axis at the focal point, is therefore equal to a distance between a film surface of a main point of the lens barrel when the lens barrel focalizes an object at the infinity. The nominal focal length takes one value in the case of a single focus lens barrel and a plurality of values in the case of a multi-focus lens barrel, and further takes a value falling within a predetermined range in the case of a zoom lens barrel.
Accuracies of controlling the exposure and the light emission of the electronic flash have been, however, increasingly demanded in recent years. With this demand, it is required that the focal length data of the lens barrel used should take a more precise value. For this purpose, when using the nominal focal length in a certain type of lens barrel, there might arise a problem in which the exposure and the light emission of the electronic flash can not be controlled at a high accuracy.
One of the causes of such a problem is that the nominal focal length of the lens barrel is not coincident with an effective focal length which will be mentioned later, and there exists the lens barrel having a large discrepancy between those values. Particularly in recent years, there has been developed a high performance zoom lens barrel having a large number of constructive lens elements, wherein the lens unit moving for zooming and the lens unit moving for focusing move in a complicated manner. Some of the above high performance zoom lens barrels have the large discrepancy between the nominal focal length and the effective focal length enough not to ignore the problem given above.
This will be explained by exemplifying the zoom lens barrel with reference to FIGS. 1A and 1B. The zoom lens barrel is constructed of four lens units L1, L2, L3 and L4. When the zoom lens barrel focalizes an object at the infinity and a certain zoom ratio, and, when the lens units L1, L2, L3 and L4 are disposed in positions as shown in FIG. 1A, the nominal focal length is a distance f between a principal point P and a focal point 0 on a film surface F.
The lens unit L2 for focusing is moved in the optical-axis direction, i.e., toward the object to focalized on the object existing in a position far at a certain finite distance at the above zoom ratio. Thereupon, the lens units L1, L2, L3 and L4 are disposed in the positions as shown in FIG. 1B. At this time, an image of the object at the finite distance is formed on the film surface F. On the other hand, the parallel light beams from the object at the infinity intersect the optical axis at a point Q on the optical axis. The effective focal length is a distance f.sub.o between the principal point P.sub.o and the point Q according to the above layout of the lens units.
The effective focal length is substantially coincident with the nominal focal length in the normal lens barrel. Some of newly developed high-performance zoom lens barrels, however, have a large discrepancy between the nominal focal length and the effective focal length.
Table 1 shows an example of calculations in two kinds of Lens barrels, i.e., a lens barrel (1) and a lens barrel (2). The lens barrel (1) is classified as a 35 to 105 mm inner focus type high-performance zoom lens barrel developed in recent years and constructed of four lens units consisting of thirteen lens elements, this lens barrel being disclosed in Japanese Patent Application Laid-Open No. 5-142475. The lens barrel (2) is a prior art front element extension type focusing zoom lens barrel constructed of four lens units, i.e., a first lens unit for focusing, a second lens unit for zooming, a third lens unit and a fourth fixed lens unit.
TABLE 1 ______________________________________ f R .beta. .beta..sub.r 1.sub.r 1 r ______________________________________ (1) 35 1.135 0.031 0.033 655.45 701.43 107 35 0.850 0.041 0.045 480.67 525.30 109 105 2.908 0.036 0.033 655.45 599.05 91 105 0.850 0.124 0.107 202.15 175.10 87 (2) 80 2.771 0.029 0.033 655.45 749.21 114 80 1.500 0.053 0.070 309.00 405.56 131 200 6.219 0.032 0.033 655.04 672.58 103 200 1.500 0.133 0.170 127.24 162.23 127 ______________________________________ f: nominal focal length mm!, R: photographic distance m!, .beta.: photographic magnification using nominal focal length f, .beta..sub.r : actual photographic magnification, 1: field range mm!using the nominal focal length f, 1.sub.r : actual field range mm!, r: 1/1.sub.r .times. 100
The prior art lens barrel (2) has a small discrepancy between the nominal focal length and the effective focal length, and hence there is substantially no problem if the photographic magnification .beta. and the field range 1 use the nominal focal length. In Table 1, r is a value given by 1/1.sub.r .times.100, and 1.sub.r represents an actual field range. When the r value is 100 or greater, this implies that no problem arises in terms of controlling, e.g., an electronic flash irradiation angle.
While in the lens barrel (1), there is a large discrepancy between the nominal focal length and the effective focal length. Accordingly, the discrepancy increases when the photographic magnification .beta. and the field range 1 use the nominal focal length. Especially when the nominal focal length is 105 mm, the r-value is under 100. This indicates that a problem is caused in terms of controlling the electronic flash irradiation angle.